Replacement of particles in a moving bed process

ABSTRACT

An apparatus for replacing particles in a process that transfers particles is disclosed. The apparatus employs a seal zone which is in communication with two zones of the process and in which particles that are being added to the process are purged. The apparatus allows particles to be replaced without reducing the normal rate of particle transfer through the process, which results in a savings in downtime costs. This invention is adaptable to a multitude of processes for the catalytic conversion of hydrocarbons in which deactivated catalyst particles are regenerated.

CROSS REFERENCE TO RELATED APPLICATION

This application is a division of prior application Ser. No. 08/239,002filed on May 6, 1994, the contents of which are incorporated herein byreference thereto now allowed U.S. Pat. No. 5,545,312.

FIELD OF THE INVENTION

The broad field of the present invention is the handling and transportof particles. The narrow field of the present invention is thereplacement of particles in processes that employ moving beds. Thepresent invention is directed toward an improved method for replacingparticles in moving bed processes having zones which contain fluids thatare inhibited from communication.

BACKGROUND OF THE INVENTION

Catalytic processes for the conversion of hydrocarbons are well knownand extensively used. Invariably the catalysts used in these processesbecome deactivated for one or more reasons. Some of these reasonsinclude the accumulation of coke deposits, the accumulation ofnonmetallic poisons such as sulfur and nitrogen, the accumulation ofmetallic poisons such as iron and lead, the agglomeration ofcatalytically-active metals such as platinum and palladium, and thedegradation of the catalyst support such as pore enlargement and phasetransformation. Depending on the nature of the catalyst and the kind anddegree of deactivation, the catalyst may become deactivated permanently.Catalyst that is neither usable nor regenerable in situ must be removedfrom the process and replaced with fresh or regenerated catalyst.

One method of replacing catalyst in a process is to stop the normaloperation of the process, unload the spent catalyst, load freshcatalyst, and then resume normal operation of the process. Althoughstraightforward, this method requires a period of time when the processis not normally operating. This period of time, called downtime, is lostproduction time that can be a significant expense in commercialhydrocarbon conversion operations.

A method that replaces the catalyst in a hydrocarbon conversion processshould have several objectives. First, it should eliminate downtime,thereby making the process more profitable. Second, it should purge thefresh catalyst prior to loading to remove contaminants that would affectadversely the performance of the process. Third, it should not require areduction in the normal or design rate of catalyst movement through theprocess. And, fourth, it should allow the process control systems thatensure reliability during normal operation to continue in operationduring the catalyst replacement.

The present invention achieves these objectives in hydrocarbonconversion processes that employ a moving bed of catalyst that istransferred between two or more zones. The present invention isparticularly useful for those processes that employ zones containingfluids that are inhibited from communication, such as a processemploying a reaction zone containing hydrocarbons and a regenerationzone containing oxygen. If the two zones are connected by a pipeline,communication between the two zones can be inhibited by closing valvesin the pipeline, but this also stops the flow of catalyst between thezones. Besides, valves that dose in pipelines containing flowingcatalyst are generally maintenance problems, because of leakage due towear from catalyst particles and malfunction due to high temperatures.

But neither closing valves nor stopping catalyst flow are requirementsfor inhibiting communication between two zones. Introducing an inertfluid into a purge, or buffer, zone between the zones can accomplishthis goal, even while catalyst is flowing between the zones. The inertfluid is generally added into a purge zone at a pressure higher than thepressure of either zone. Beginning at the point of introduction of theinert fluid, two portions of the inert fluid flow in oppositedirections--one portion toward one zone and the other toward the otherzone. Therefore, relative to the flow of catalyst, one portion of theinert fluid flows countercurrently whereas the other portion flowsconcurrently. The two portions of the inert fluid effectively purge thecatalyst between the two zones and inhibit the communication of the twofluids.

In moving bed processes that employ a purge zone like that justdescribed, the present invention uses the existing purge zone vessel andits associated process control instrumentation to replace the catalyst.Therefore, in addition to achieving the objectives described above, thepresent invention maximizes the use of existing equipment and minimizesthe need for additional equipment that would sit idle except duringcatalyst replacement. For these reasons, this invention is a simple,efficient, and cost-effective method of replacing catalyst in moving bedprocesses.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for replacingparticles in a process that transfers solid particles.

In a broad embodiment, the method comprises withdrawing a first streamcomprising particles from a first zone. This first stream is rejectedfrom the process. A second stream comprising particles and a first fluidis introduced to a second zone, and the particles are passed through thesecond zone. A third stream comprising a second fluid is passed to thesecond zone at a rate that is sufficient to purge the first fluid fromthe total void volume in said second zone. A fourth stream comprisingthe first fluid and the second fluid is withdrawn from the second zoneand passed to the first zone. A fifth stream comprising particles andthe second fluid is withdrawn from the second zone. The fifth stream ispassed to a third zone that is in uninterrupted communication with thesecond zone.

In a more detailed embodiment, the method comprises a process in whichcatalyst particles are passed from a first zone to a second zone andfrom the second zone to a third zone. The passage of catalyst particlesfrom the first zone to the second zone is interrupted. A first streamcomprising catalyst particles is withdrawn from the first zone. Thisfirst stream is rejected from the process. A second stream comprisingcatalyst particles and oxygen is introduced to the second zone, and thecatalyst particles are passed through said second zone. A third streamcomprising nitrogen is passed to the second zone at a rate that issufficient to purge oxygen from the total void volume in the secondzone. A fourth stream comprising oxygen and nitrogen is withdrawn fromthe second zone and passed to the first zone. A fifth stream comprisingcatalyst particles and nitrogen is withdrawn from the second zone. Thefifth stream is passed to a third zone that is in uninterruptedcommunication with the second zone.

The apparatus allows for the replacement of particles in a process forthe transfer of particles. The apparatus comprises three vessels. Avertically-positioned first vessel has a first particle outlet. Avertically-positioned second vessel is located below the first vessel.The second vessel has a first particle inlet and a second particleoutlet, and particles may move by gravity from the first particle inletto the second particle outlet. The second vessel defines a lower portionfor containing particles. The second vessel also defines a means for gasaddition, means to distribute gas about the lower portion of the secondvessel for contact with particles in the second vessel, and a gasoutlet. A vertically-positioned third vessel is located below the secondvessel and has a second particle inlet. A vertically-extended firstconduit for containing a gas flow restriction comprising a bed ofparticles is in communication with the first particle outlet to removeparticles therefrom. A vertically-extended second conduit is incommunication with the first conduit to remove particles therefrom andalso with the first particle inlet to supply particles thereto. A meansallows for communicating gas from the gas outlet to the first vessel andfor restricting the flow of gas from the gas outlet. Avertically-extended third conduit is in communication with the secondparticle outlet to remove particles therefrom and also with the secondparticle inlet to supply particles thereto. A means allows fordischarging particles from the first vessel. A third particle inlet isin communication with the second vessel.

Other objects, embodiments, and details of the present invention arepresented in the following detailed description of the presentinvention.

BRIEF DESCRIPTION OF THE DRAWING

The drawing is a schematic flow diagram of a preferred embodiment of themethod of this invention of replacing solid particles in a process thattransfers solid particles. The drawing is a schematic flow diagram ofthe method of replacing solid particles in which only those lines,valves, etc., relevant to the flow of catalyst particles are shown.

DETAILED DESCRIPTION OF THE INVENTION

In its broadest terms, the present invention can be used to replacesolid particles in a multitude of processes that transfer particlesbetween any two zones which contain fluids that are inhibited fromcommunication. One such application that requires inhibitedcommunication between the fluids of two zones is the transfer ofcatalyst between an oxygen-containing regeneration zone and ahydrocarbon-containing reaction zone. Other applications includetransferring noble-metal catalyst between an oxygen-containingregeneration zone and a hydrogen-containing zone which reduces the noblemetals, and transferring catalyst between an oxygen-containingregeneration zone and a hydrogen-containing zone which transports thecatalyst to the hydrocarbon-containing reaction zone. In theseapplications, inhibiting communication is necessary because the fluidsof the two zones could react with each other to form water. The presenceof water in either the reaction zone or the regeneration zone affectsthe performance of many hydrocarbon conversion catalysts.

Systems having a hydrocarbon-containing reaction zone and anoxygen-containing regeneration zone, as applied to petroleum refining,may be employed in a wide variety of hydrocarbon conversion reactionsincluding catalytic reforming, fixed-bed alkylation, hydrorefining,hydrocracking, dehydrogenation, hydrogenation, steam reforming, etc. Thecatalysts used in these processes are maintained in one or morehydrocarbon-containing zones. Over time, the catalyst in the reactionzone(s) generally becomes deactivated because of the accumulation ofcoke deposes. Regeneration of the catalyst to remove the coke depositshelps restore the activity of the catalyst. Coke deposits are generallyremoved from the catalyst by contacting the catalyst with anoxygen-containing gas to combust and remove the coke in a regenerationzone. Many of these processes use a reaction zone and a regenerationzone in side-by-side relation to each other. In these systems, thecatalyst is continuously or semi-continuously removed from the reactionzone and transferred to the regeneration zone for coke removal.Following coke removal, the catalyst is removed from the regenerationzone and transferred back to the reaction zone. Therefore, in thesewell-known and widely-practiced systems, there is a requirement totransfer the catalyst back and forth between a hydrocarbon-containingzone and an oxygen-containing zone without communication or cross-mixingof the atmospheres of the two zones.

Although a regeneration zone that removes coke by combustion usingoxygen has a generally beneficial effect on the catalyst, thecombination of extreme combustion temperatures and the water formedduring combustion can cause some degree of permanent damage to thecatalyst, such as a reduction in the catalyst's surface area. The degreeof permanent damage that results from a single regeneration may beinsignificant to the catalyst performance, activity, and physicalproperties. But, the cumulative effect on physical properties of tens orhundreds of successive regenerations can gradually lead to adeterioration in catalyst performance and permanent deactivation. Whenthe losses that result from a deterioration in performance outweigh thecost of replacing the catalyst inventory, economics generally dictatethat the catalyst be replaced. The present invention allows thisreplacement to be done without having to stop the normal operation ofthe reaction section. Thus, when using this invention, the cost ofreplacing catalyst is significantly reduced because loss of productiondue to downtime of the reaction section does not compound the cost ofcatalyst replacement.

The present invention is applicable to a wide variety of hydrocarbonconversion processes including hydrogenation and dehydrogenationprocesses, but the most widely practiced hydrocarbon conversion processto which the present invention is applicable is catalytic reforming.Therefore the discussion of the invention contained herein will be inreference to its application to a catalytic reforming reaction system.It is not intended that such discussion limit the scope of the inventionas set forth in the claims.

Catalytic reforming is a well-established hydrocarbon conversion processemployed in the petroleum refining industry for improving the octanequality of hydrocarbon feedstocks, the primary product of reformingbeing motor gasoline. The art of catalytic reforming is well known anddoes not require detailed description herein.

Briefly, in catalytic reforming, a feedstock is admixed with a recyclestream comprising hydrogen and contacted with catalyst in a reactionzone. The usual feedstock for catalytic reforming is a petroleumfraction known as naphtha and having an initial boiling point of about180° F. and an end boiling point of about 400° F. The catalyticreforming process is particularly applicable to the treatment ofstraight run gasolines comprised of relatively large concentrations ofnaphthenic and substantially straight chain paraffinic hydrocarbons,which are subject to aromatization through dehydrogenation and/orcyclization reactions.

Reforming may be defined as the total effect produced by dehydrogenationof cyclohexanes and dehydroisomerization of alkylcyclopentanes to yieldaromatics, dehydrogenation of paraffins to yield olefins,dehydrocyclization of paraffins and olefins to yield aromatics,isomerization of n-paraffins, isomerization of alkylcycloparaffins toyield cyclohexanes, isomerization of substituted aromatics, andhydrocracking of paraffins. Further information on reforming processesmay be found in, for example, U.S. Pat. No. 4,119,526 (Peters et al.);U.S. Pat. No. 4,409,095 (Peters); and U.S. Pat. No. 4,440.626 (Winter etel.).

A catalytic reforming reaction is normally effected in the presence ofcatalyst particles comprised of one or more Group VIII noble metals(e.g., platinum, iridium, rhodium, palladium) and a halogen combinedwith a porous carrier, such as a refractory inorganic oxide. The halogenis normally chlorine. Alumina is a commonly used carrier. The preferredalumina materials are known as the gamma, eta and theta alumina withgamma and eta alumina giving the best results. An important propertyrelated to the performance of the catalyst is the surface area of thecarrier. Preferably, the carrier will have a surface area of from 100 toabout 500 m² /g. The particles are usually spheroidal and have adiameter of from about 1/16th to about 1/8th inch (1.5-3.1 mm), thoughthey may be as large as 1/4th inch (6.35 mm). In a particularregenerator, however, it is desirable to use catalyst particles whichfall in a relatively narrow size range. A preferred catalyst particlediameter is 1/16th inch (3.1 mm). During the course of a reformingreaction, catalyst particles become deactivated as a result ofmechanisms such as the deposition of coke on the particles; that is,after a period of time in use, the ability of catalyst particles topromote reforming reactions decreases to the point that the catalyst isno longer useful. The catalyst must be reconditioned, or regenerated,before it can be reused in a reforming process.

In preferred form, the reformer will employ a moving bed reaction zoneand regeneration zone. The present invention is applicable to a movingbed regeneration zone. Fresh catalyst particles are fed to a reactionzone, which may be comprised of several sub-zones, and the particlesflow through the zone by gravity. Catalyst is withdrawn from the bottomof the reaction zone and transported to a regeneration zone where ahereinafter described multi-step regeneration process is used torecondition the catalyst to restore its full reaction promoting ability.Catalyst flows by gravity through the various regeneration steps andthen is withdrawn from the regeneration zone and furnished to thereaction zone. Movement of catalyst through the zones is often referredto as continuous though, in practice, it is semi-continuous. Bysemi-continuous movement is meant the repeated transfer of relativelysmall amounts of catalyst at closely spaced points in time. For example,one batch per minute may be withdrawn from the bottom of a reaction zoneand withdrawal may take one-half minute, that is, catalyst will flow forone-half minute. If the inventory in the reaction zone is large, thecatalyst bed may be considered to be continuously moving. A moving bedsystem has the advantage of maintaining production while the catalyst isremoved and replaced.

The majority of the description of the present invention is presented interms of transferring catalyst particles from a regeneration zonecontaining oxygen, to a purge zone containing nitrogen, and to areaction zone containing hydrocarbon. However, this description is notintended to limit the scope of the invention to this particulararrangement.

The drawing shows a method for the replacement of catalyst particles ina process that transfers catalyst particles between a regeneration zone10, a seal zone 60, and a reaction zone 70. In the process, catalystparticles are transferred from the regeneration zone 10 to the seal zone60, and then to the reaction zone 70. The method of replacement ofcatalyst particles comprises withdrawing spent catalyst particles fromthe regeneration zone 10 and adding fresh catalyst particles into theseal zone 60. The regeneration zone 10, seal zone 60, and the reactionzone 70 are shown in a relation where the regeneration zone 10 is abovethe seal zone 60, which is above the reaction zone 70. The regenerationzone 10 and the reaction zone 70 may be maintained independently withtheir own atmospheres and at their own pressures by any suitable means,and such means are not essential elements of the method.

The withdrawal of spent catalyst particles from the regeneration zone 10may be by any suitable method. Gases that are entrained with thewithdrawn catalyst may be purged from the catalyst prior to withdrawal,but this purging is not an essential step of this invention. Preferably,purging of entrained gases from the withdrawn catalyst particles is notrequired. The withdrawal may be continuous or semi-continuous. Thedrawing shows a preferred method of withdrawing spent catalyst particlesfrom a regeneration zone that operates at or above atmospheric pressureand contains oxygen gas. Spent catalyst particles are withdrawn from theregeneration zone 10 through line 11, line 14, valve 16, line 18, arestriction 20, and line 22. The flow of catalyst particles may be bygravity and it may be assisted by a difference in pressure between theregeneration zone 10 and the ambient atmosphere. Exiting from theregeneration zone 10 with the catalyst particles are regeneration gasessuch as oxygen that escape through the open lines when the valve 16 isopened. Preferably, the combined stream of exiting catalyst particlesand regeneration gases is a moving pecked bed of catalyst particlesbetween the regeneration zone 10 and the restriction 20. The catalystparticles exiting the line 22 may be transported away to off-siteprocessing by any suitable means, such as in drums or bags, and suchmeans is not an essential element of the method.

The restriction 20 limits the rate of withdrawal of the catalystparticles to either a convenient rate for transporting away thewithdrawn catalyst particles or to the design flow rate of catalystparticles through the regeneration zone 10, whichever is smaller. Inaddition, the restriction 20 may also limit the rate of escape of theregeneration gases to a rate that does not adversely affect the normaloperation of the regeneration zone 10. If the regeneration zone operatesat a pressure greater than the ambient pressure, the means for reducingthe pressure surrounding the withdrawn catalyst particles from thepressure of the regeneration zone 10 to ambient pressure may be therestriction 20, with most of this reduction in pressure may be takenacross the restriction 20. Preferably, however, the restriction 20 isthe means for maintaining a moving packed bed of catalyst particles inthe withdrawal lines 11, 14, and 18 and in the valve 16, and thereduction in pressure occurs substantially uniformly across thewithdrawal lines 11, 14, and 18, the valve 16, and the restriction 20.Those skilled in the art of particle transport are capable of designingthe size of the opening of the restriction 20, and the sizes and lengthsof the lines 11, 14, and 18 and the valve 16 to achieve the desired flowrates of catalyst particles and regeneration gases.

Means for indirect or direct heat exchange may be used to cool thewithdrawn catalyst particles, if the temperature of the withdrawncatalyst particles are too hot for handling. An example of indirect heatexchange is a shell-and-tube heat exchanger through which the catalystparticles flow on the tube side and a cooling fluid flows on the shellside. If the temperature of the withdrawn catalyst particles are greaterthan the ambient temperature, the simplest shell-and-tube heat exchangerwould be a conduit, such as line 14, 18, or 22, containing the hotcatalyst particles and ambient air surrounding the outside of theconduit. Heat transfer from the conduit to the ambient air may be bynatural convection or forced convection. Those skilled in the art ofheat transfer are capable of designing a conduit that would be suitablefor sufficiently cooling the catalyst particles. An example of directheat exchange is introducing ambient air into one of the lines throughwhich the catalyst particles are withdrawn, preferably line 14. Thecatalyst particles are cooled by direct heat transfer to the air, andthe air may be disengaged from the withdrawn catalyst particles at theoutlet of line 22.

The addition of fresh catalyst particles into the seal zone 60 may be byany suitable method. Gases that are entrained with the fresh catalystmay be purged from the catalyst prior to entering the seal zone 60, butthis purging is not an essential step of this invention. Preferably,purging of entrained gases from the fresh catalyst particles is notrequired before fresh the fresh catalyst particles enter the seal zone60. The addition of fresh catalyst particles may be continuous orsemi-continuous. The drawing shows a preferred, batch-wise method ofadding fresh catalyst particles into a seal zone 80 that operatesslightly or substantially above atmospheric pressure. Fresh catalystparticles are added into the seal zone 60 via an addition funnel 40 andan addition hopper 50. The drawing shows the addition funnel 40, theaddition hopper 50, and the seal zone 60 in a preferred relation wherethe addition funnel 40 is above the addition hopper 50, which is aboveand axially offset from the seal zone 60. The catalyst particles thatare loaded into the addition funnel 40 may be transported to the funnelby any suitable means, such as in drums or bags, and such means is notan essential element of the method. The flow of catalyst particles fromthe addition funnel 40 into the addition hopper 50 may be by gravity.The flow of catalyst particles from the addition hopper 50 into the sealzone 60 may be by gravity or it may be assisted by a difference inpressure between the addition hopper 50 and the seal zone 60.

Referring to the drawing, the preferred method of addition of freshcatalyst particles into the seal zone 60 is a 4-step cycle ofdepressuring, loading, pressuring, and unloading the addition hopper 50.Initially, the addition funnel 40 and the addition hopper 50 are emptyand the four valves 47, 36, 48, and 54 that control flow to and from theaddition hopper 50 are closed. First, valve 47 is opened and theaddition hopper 50 is depressured to atmosphere through line 45, valve47, and line 49. Second, valve 47 is closed, valve 36 is opened, freshcatalyst particles are placed in the addition funnel 40, and theparticles fill the addition hopper 50 through line 38, valve 36, andline 42. Third, valve 36 is closed, valve 48 is opened, and the additionhopper 50 is pressured with nitrogen through line 46, valve 48, and line44. Preferably, the source of the nitrogen is the plant supply ofcompressed nitrogen, and the addition hopper 50 is pressured to apressure that is about equal to the pressure of a surge bed 57 in anupper chamber of the seal zone 60. Fourth, valve 48 is closed, valve 54is opened, and the addition hopper 50 is unloaded into the seal zone 60through line 52, vane 54, line 56, and line 33. Finally, valve 54 isclosed, and the addition cycle may be started over again. The cycle maybe automated into a logical sequence that can be controlled at least inpart by an automatic, programmable controller. Those skilled in the artof particle transfer are capable of adding the necessary levelinstruments, valve actuators, valve position indicators, and timers toautomate the sequence.

The seal zone 60 contains three catalyst particles beds--thepreviously-mentioned surge bed 57, a purge bed 61, and an outlet bed 63.The drawing shows the three beds in a preferred relation where the surgebed 57 is above the purge bed 61, which is above the outlet bed 63.Preferably, the three beds are cylindrical, pecked, moving beds. Freshcatalyst particles from the addition hopper 50 enter the surge bed 57.Catalyst particles descend by gravity flow from the surge bed 57,through the purge bed 61, through the outlet bed 63, and out of the sealzone 60.

The surge bed 57 is a reserve or surge volume of catalyst particles forsupplying the purge bed 61. During catalyst transfer from theregenerator 10 to the seal zone 60, the inventory of catalyst particlesin the surge bed 57 is essentially constant, even though catalystparticles flow through the surge bed 57. The inventory of catalystparticles during catalyst transfer is the amount of catalyst in themoving packed bed of catalyst that extends from the bottom of the line35 to the top of the purge bed 61. But, during catalyst replacement,when catalyst particles are being transferred from the addition hopper50 to the seal zone 60 according to the addition cycle describedpreviously, the inventory of catalyst in the surge bed 57 may vary.Preferably, when valve 54 opens, catalyst particles drain completelyfrom the addition hopper 50, the line 52, and the valve 54. Morepreferably, the catalyst particles drain completely from line 56, line33, and line 35, too. This ensures that them will be no catalystparticles in valve 54 that could damage valve 54 when valve 54 closes.In order for line 35 to completely drain into the surge bed 57, thelevel of surge bed 57 must be sufficiently low to receive at least thequantity of catalyst in addition hopper 50 before vane 54 opens.Therefore, it is preferred that during catalyst replacement theinventory of catalyst particles in the surge bed 57 increases whencatalyst particles are unloaded into the seal zone 60 and decreasesduring the other steps of the addition cycle. This surge volume may alsobe sized large enough to accommodate unexpected interruptions orvariations in the addition cycle or in the rate of withdrawal ofcatalyst particles from the surge bed 57. A level instrument may beprovided to monitor the variations in the inventory of the surge bed 57during catalyst replacement.

Of course, the addition process shown in the drawing is intermittentbecause it is a batch-wise process, but the addition is not limited tointermittent processes. Likewise, the withdrawal process shown in thedrawing may be continuous if the valve 16 is kept open, however thewithdrawal may also be intermittent if the valve 16 is closed from timeto time. Thus, at a given moment during catalyst replacement, theaddition rate and the withdrawal rate may be unequal. Preferably,however, the rate of addition of catalyst particles into the seal zone60 is not less than the rate of withdrawal of catalyst particles fromthe seal zone 60, in order to maintain the inventory of catalystparticles in the surge bed 57 either substantially constant or within apredetermined preferred inventory range. Furthermore, it is alsopreferable that the rate of addition of catalyst particles into the sealzone 60 is not less than the rate of withdrawal of catalyst particlesfrom the regeneration zone 10, in order to maintain the combinedinventory of catalyst particles in the regeneration zone 10, the sealzone 60, and the reaction zone 70 either substantially constant orwithin a predetermined preferred inventory range. The terms "rate ofaddition" or "rate of withdrawal" may refer to an instantaneous rate orto an average rate over a predetermined or convenient time period.

Entering the surge bed 57 with the fresh catalyst particles from theaddition hopper 50 may be ambient gases such as oxygen to which thefresh particles were exposed during transport to or addition into theaddition funnel 40. A purge stream comprising nitrogen enters the sealzone 60 in order to permit the catalyst particles that have descendedinto the purge bed 61 from the surge bed 57 to be purged of oxygen tothe desired degree. The nitrogen enters the seal zone 60 through line82, regulating valve 64, and line 68. Control of the amount of nitrogenthat enters is by means of the regulating valve 64 operated ondifferential pressure control by signal selector 80. The source of thenitrogen is preferably the plant supply of compressed nitrogen.

The nitrogen entering the seal zone 60 through line 66 is preferablydistributed uniformly upwardly in a counterflow manner through the purgebed 61 from an annular chamber 59. Annular chamber 59 is defined by thewall of the seal zone 60 and a baffle having a vertically-extendedcylindrical section 65 that is concentrically located with respect tothe seal zone 60. The upper portion of the baffle consists of afrusto-conical section 58 that is attached to the seal zone and supportsthe upper end of cylindrical section 65. The bottom of the cylindricalsection 65 is open and allows gas to be distributed about the entirecircumference of the annular chamber 59 and the purge bed 61. Thenitrogen passes into the annular chamber 59 which distributes itdownwardly and then uniformly upwardly into the purge bed 61 incounterflow relation to the downward movement of the catalyst particles.The distributor may be of any suitable design, such as perforated pipesor conduits or slot-type channels, provided that the distributorinhibits fluidization, attrition, or breakage of catalyst particles thatpass nearby. A bed of catalyst particles may act as a distributor, andwhere a cylindrical bed is used for this purpose in the purge bed 61,the bed has a length:diameter ratio that is at least one. After purging,the catalyst particles descend from the purge bed 61 into the outlet bed63. The outlet bed 63 is a volume of catalyst particles for supplyingthe reaction zone 70. The catalyst particles exit the seal zone 60through a line 68 and into the reaction zone 70.

The nitrogen that enters the seal zone 60 exits in two portions. Fromthe preceding description, one portion flows from the annular chamber 59upwardly through the purge bed 61. This first portion flows through thesurge bed 57 and exits the seal zone 60 through line 28. It passes intothe regeneration zone 10 through valve 26, and lines 24, 12, and 11. Theother portion flows from the annular chamber 59 downwardly through theoutlet bed 63 and exits the seal zone 60 through line 68. This secondportion passes into the reaction zone 70.

One consequence of the flow of nitrogen from the seal zone 60 to theregeneration zone 10 is that the annular chamber 59 operates at apressure that is not less than the pressure of the regeneration zone 10.The pressure of the annular chamber 59 is preferably not less than 2inches H₂ O greater than the pressure of the regeneration zone 10. Morepreferably, the pressure of the annular chamber 59 is not less than 10inches H₂ O greater than the pressure of the regeneration zone 10. Thepressure difference between the regeneration zone 10 and the annularchamber 59 is measured by a differential pressure measuring instrument74 that is in communication with the regeneration zone 10 through apressure tap 72 and with the annular chamber 59 through a pressure tap76. A set-point that corresponds to the desired differential pressure ispresent in the instrument 74. The instrument 74 provides an outputsignal 78 that corresponds to the difference between the actualdifferential pressure and the desired differential pressure across thebeds 61 and 57, valve 26, and lines 28, 24, 12, and 11.

From the preceding description, a stream of catalyst particles andnitrogen passes from the outlet bed 63 of the seal zone 60, through line68, and into the reaction zone 70. As a result of the flow of thisstream, the pressure of the reaction zone 70 is less than the pressureof the annular chamber 59. The pressure of the reaction zone 70 ispreferably not less than 10 inches H₂ O less than the pressure of theannular chamber 59. The pressure difference between the annular chamber59 and the reaction zone 70 is measured by a differential pressuremeasuring instrument 63 that is in communication with the annularchamber 59 through a pressure tap 86 and with the reaction zone 70through a pressure tap 90. A set-point that corresponds to the desireddifferential pressure is present in the instrument 88. The instrument 88provides an output signal 84 that corresponds to the difference betweenthe actual differential pressure and the desired differential pressureacross the beds 63 and the line 68.

In a preferred method of operation, these two pressure differences--onebetween the annular chamber 59 and the regeneration zone 10, and theother between the annular chamber 59 and the reaction zone 70--aremaintained at or near to their desired values by a control system, asshown in the drawing. The output signals 78 and 84 enter a signalselector 80 which compares the signals and produces an output signal 82.The output signal 82 corresponds to the signal, either 78 or 84, thatindicates the greater requirement for nitrogen to reestablish thedesired pressure difference. In this way, the output signal 82 adjuststhe position of regulating valve 64 according to the differentialpressure measuring instrument that indicates the requirement for thegreater inflow of nitrogen. This ensures that the nitrogen flow throughregulating valve 64 is sufficient to satisfy the desired differentialpressures.

The regeneration zone 10 communicates catalyst to the seal zone 60 byline 11, line 12, line 30, valve 32, line 34, line 33, and line 35.These lines are a means for transferring catalyst particles from theregenerator 10 to the seal zone 60 by opening valve 32, and transferringcatalyst particles by gravity flow. Valve 32 is an example of a means tointerrupt the communication of catalyst particles through lines 30 and34. When valve 32 is open, it preferably provides uninterruptedcommunication of catalyst particles through lines 30 and 34, withoutattriting, chipping, or otherwise damaging the catalyst particles.Valves that are suitable for this service are commercially-available,and those skilled in the art of particle transport are able to select asuitable valve. During catalyst particle replacement, valve 32 is closedand the catalyst flow from the regeneration zone 10 is not through line12, but instead through line 14 and out of the process. During catalystparticle replacement, the catalyst flow into the seal zone 60 is notthrough lines 30 and 34, but instead through line 56 from the additionhopper 50. Preferably, the catalyst particles in line 12 form a fixedpacked bed during catalyst particle replacement, but when catalystparticles are being transferred from the regeneration zone 10 into theseal zone 60 the particles in line 12 form a moving packed bed.

From the preceding description, during catalyst particle replacement,nitrogen flows upward through beds 61 and 57, valve 26, and lines 28,24, 12 and 11, because valve 26 is open. And catalyst particles do notflow from the regeneration zone 10 downward through lines 11, 12, 30,34, 33, and 35 because valve 32 is closed. However, catalyst particlesalso do not flow downward into the seal zone 60 through lines 12, 24 and28, and valve 26. Those skilled in the art of particle transport areable to design the Junction of lines 12, 24 and 30 so that when valve 32is closed and valve 26 is open, nitrogen flows upward through lines 28,24, and 12, but catalyst particles do not flow in the reverse directionand downward through the same lines. As shown in the drawing, this ispreferably achieved by installing lines 12 and 30 in a vertical ornearly-vertical orientation, and intersecting line 24 into lines 12 and30 on a downward slope. The angle that line 24 makes with the horizontalis at least equal to the angle of repose of the catalyst particles, andpreferably the angle is 10 or more degrees greater than the angle ofrepose. The length of the downward sloping section of line 24 is atleast 1 foot (0.3 meter) and preferably at least 2 feet (0.6 meter).

From the preceding description, during catalyst particle replacement,the differential pressure measuring instrument 74 measures thedifferential pressure between the annular chamber 59 and theregeneration zone 10, and this differential pressure is measured asnitrogen flows upward through the beds 61 and 57, valve 26, and lines28, 24, 12 and 11. At times other than during catalyst particlereplacement, valve 26 is closed and valve 32 is open to allow catalystparticles to be transferred from the regeneration zone 10 to the sealzone 60. Therefore, when catalyst particles are being transferred fromthe regeneration zone 10 to the seal zone 60, instrument 74 measures thedifferential pressure as nitrogen flows upward through the beds 61 and57, valve 32, and the lines 35, 33, 34, 30, 12 and 11. Both duringcatalyst particle replacement and during catalyst transfer between theregeneration zone 10 and the seal zone 60, the differential pressurethat instrument 74 measures includes the differential pressure acrossthe beds 61 and 57 and the lines 11 and 12. During catalyst particlereplacement, the measured differential pressure includes the pressuredifference across valve 26 and lines 24 and 28, all of which contain nocatalyst particles. But during catalyst particle transfer from theregeneration zone 10 to the seal zone 60, the measured differentialpressure includes the pressure difference across valve 32 and lines 30,34, 33, and 35, which contain a packed bed of catalyst particles.

All other factors being equal, then, the instrument 74 measures asomewhat-lower differential pressure during catalyst particlereplacement than during catalyst transfer because of the lowerdifferential pressure across the catalyst-free valve 26 and lines 24 and28 in comparison to across the catalyst-containing valve 32 and lines30, 34, 33, and 35. In order to minimize the difference in measureddifferential pressures during these two modes of operation, the lengthsof the lines 30, 34, and 33 are kept to a minimum. Although the lengthof line 35 is also kept to a minimum, its length is generally not lessthan 1 foot (0.3 meter). This prevents the top of the surge bed 57 fromgetting too near to the gas outlet, and this helps to prevent catalystparticles from being carded upward into the line 28. Preferably, thecombined length of lines 11 and 12 is at least 50% of the total lengthof the lines 11, 12, 30, 34, 33, and 35. More preferably, the combinedlength of lines 11 and 12 is at least 80% of the total length of lines11, 12, 30, 34, 33, and 35. Alternatively, during catalyst particletransfer from the regeneration zone 10 to the seat zone 60 when valve 32is open and valve 26 is closed, the differential pressure across lines11 and 12 is at least 50% of the total differential pressure across beds61 and 57, valve 32, and lines 11, 12, 30, 34, 33 and 35. Morepreferably, the differential pressure across lines 11 and 12 is at least80% of the total differential pressure across beds 61 and 57, valve 32,and lines 11, 12, 30, 34, 33, and 35.

The packed bed of catalyst particles in the lines 12 and 11 is apreferred means to restrict the gas flow from the seal zone 60 to theregeneration zone 10. The packed bed may be moving or fixed. However,this invention is not limited to a flow restriction comprising a packedbed of catalyst particles, and the means to restrict the gas flow maycomprise a restriction orifice, baffles, or any other restriction.Preferably, the restriction does not cause attriting, chipping, or anydamage to the catalyst particles.

Where lines 11 and 12 are conduits and the means to restrict the gasflow is a packed bed of catalyst particles in the conduits, additionalmeans must be provided to hold the catalyst particles in the conduitsboth at the lower and upper ends of the conduits. During catalysttransfer the catalyst particles in the surge bed 57 preferably fill theupper chamber of the seal zone 60 up to the bottom of the line 35 in theseal zone 60, and the bed of catalyst particles in the surge bed 57prevents lines 30, 34, 33 and 35 from draining empty of catalyst,thereby holding the catalyst particles in the line 12. During catalystreplacement, valve 32, which is then closed, and thepreviously-described angled intersection of line 24 into the junction oflines 30 and 12, together prevent line 12 from draining empty, therebyholding catalyst particles in line 12 at the bottom of line 12. Duringboth catalyst transfer and catalyst particle replacement, the upwardflow of gas through the conduits is low enough so as not to fluidize thecatalyst particles and carry them out the top of line 12. Moreover, thepacked bed of catalyst in the regeneration zone 10 communicates catalystparticles to line 11. Line 11, in turn, communicates catalyst particlesto line 12 and is sufficient to provide enough static head to maintainthe particles in at the top of line 12. Other less-preferred means tohold the catalyst particles can also be used, such as baffles or plates,at the top, at the bottom, and within the lines 11 and 12.

During catalyst particle replacement, this invention is not limited topassage of gas from the seal zone 60 into the regeneration zone 10through the same lines --e.g., lines 11 and 12--through which thecatalyst exits the regeneration zone 10 during catalyst transfer. Thegas that exits the seal zone 60 may flow through an entirely separateconduit into the regeneration zone 10. Such a conduit would also requirea flow restriction for gas flow, and where the flow restriction is apacked bed of particles, the conduit would require means to hold thecatalyst particles in the conduit. In this variation of the invention, aseparate conduit is in gas communication with both the regeneration zone10 and with the seal zone 60, contains a packed bed of catalystparticles, and employs the previously-described means for holding thecatalyst particles in the conduit. In this variation, the conduit is notused for catalyst transfer, but is instead employed as a flowrestriction for the gas that flows from the seal zone 60 to theregeneration zone 10.

In keeping with the preference that the combined length of lines 30, 34,33 and 35 be kept to a minimum, the length of the flow path of catalystparticles through valve 32 is preferably kept to a minimum, too. Valvesthat are commercially-available and suitable for this service typicallyhave a length of flow path of between 0.5-1.5 feet (0.15-0.45 meter) anda maximum dimension of the cross-section of the flow path between 2-8inches (0.05-0.2 meter). Where conduits are used for lines 30, 34, and33, preferably the length of the conduits 30, 34, and 33 are kept to aminimum, not exceeding 3 feet (0.9 meter).

One other factor--the quantity of catalyst particles in the surge bed57--also results in the differential pressure instrument 74 measuring asomewhat-lower differential pressure during catalyst particlereplacement than during catalyst transfer, but since its effect isgenerally not significant it is described only in passing. Duringcatalyst transfer, the surge bed 57 preferably fills the upper chamberof the seal zone 60 up to the inlet of line 35 as a packed bed ofcatalyst particles. But during catalyst particle replacement, the surgebed 57 may not always fill the upper chamber, in part because theaddition of catalyst particles from the hopper 50 may be intermittent.All other factors being equal, then, the instrument 74 measures asomewhat-lower differential pressure during catalyst particlereplacement than during catalyst transfer in part because of the lowerdifferential pressure across the smaller surge bed 57. This effect isgenerally not significant because the cross-section of the suge bed 57is relatively large in comparison to the cross-section of the purge bed61 or of the lines 11 and 12. Thus, variations in the catalyst inventoryof the surge bed 57 have a relatively small effect on the differentialpressure measured by the instrument 74.

The flow rate of the nitrogen through the purge bed 61 is preferably ata rate less than that effective to terminate the flow of catalystparticles through the purge bed 61, thereby allowing the catalystparticles to flow at least intermittently through the purge bed 61.Moreover, the flow rate of the nitrogen through the purge bed 61 ispreferably at a rate less than that effective to fluidize the catalystin the purge bed 61. In addition, the flow rate of the purge stream isnot less than that effective to purge oxygen from the total void volumein the purge bed 61. The total void volume in the purge bed 61 isdefined as the volume of the pores within the catalyst particles plusthe voidage volume between the catalyst particles in the purge bed 61.The physical characteristics of the catalyst determine the volume of thepores within the catalyst particles, and the voidage volume between thecatalyst particles depends on how densely the catalyst particles arepacked in the purge bed 61. Since the rate at which the total voidvolume enters the purge bed 61 depends on the rate of flow of thecatalyst particles, the flow rate of the purge stream that is effectiveto purge oxygen from the total void volume depends on the rate of flowof the entering catalyst particles. Preferably, the ratio of the volumeof purge stream to the total void volume entering the purge bed 61 isgreater than 1.0, provided that the purge stream does not interfere withthe flow of catalyst particles as previously described in thisparagraph. Depending on the physical characteristics of the catalyst,the ratio of the volume of purge stream to the total void volumeentering the purge bed 61 may be between 2.5 and 3.5. Preferably, theresidence time of the catalyst particles in the purge zone 61 is between0.1 and 60 minutes, and more preferably between 0.5 and 30 minutes. Thegas stream that exits the purge bed 61 comprises nitrogen and oxygen andis passed from the seal zone 60 into the regeneration zone 10. Thenitrogen is ultimately rejected from the regeneration zone 10 in anysuitable stream, typically the regeneration zone vent stream.

What is claimed is:
 1. An apparatus for the transfer of particles, saidapparatus comprising:(a) a vertically-positioned first vessel having afirst particle outlet; (b) a vertically-positioned second vessel, whichis located below said first vessel, having a first particle inlet and asecond particle outlet, and through which particles may move by gravityfrom said first particle inlet to said second particle outlet, and whichdefines a lower portion for containing particles, a means for gasaddition, means to distribute gas about said lower portion of saidsecond vessel for contact with particles in said second vessel, and agas outlet; (c) a vertically-positioned third vessel, which is locatedbelow said second vessel having a second particle inlet, (d) avertically-extended first conduit in communication with said firstparticle outlet to remove particles therefrom and for containing a gasflow restriction comprising a bed of particles; (e) avertically-extended second conduit in communication with said firstconduit to remove particles therefrom and in communication with saidfirst particle inlet to supply particles thereto; (f) means forcommunicating gas from said gas outlet to said first vessel andrestricting the flow of gas from said gas outlet; (g) avertically-extended third conduit in communication with said secondparticle outlet to remove particles therefrom and in communication withsaid second particle inlet to supply particles thereto; (h) means fordischarging particles from said first vessel; and (i) a third particleinlet in communication with said second vessel.
 2. The apparatus ofclaim 1 wherein said second conduit comprises a means to interrupt saidcommunication of particles from said first conduit.
 3. The apparatus ofclaim 1 wherein said means for discharging particles comprises a thirdparticle outlet defined by said first conduit.
 4. The apparatus of claim1 wherein said third particle inlet is defined by said second conduit.5. The apparatus of claim 1 wherein said means for discharging particlescomprises a third conduit that is in communication with said firstvessel and for containing a gas flow restriction comprising a bed ofparticles and said gas outlet is in communication with said thirdconduit.
 6. The apparatus of claim 1 wherein said second vessel has acylindrical outer wall and said means to distribute gas about said lowerportion of the second vessel includes a cylindrical inner wall having alower end, said inner wall being spaced from said outer wail of saidsecond vessel to define an annular chamber which distributes gas aboutsaid lower end of said inner wall.
 7. The apparatus of claim 1 furthercharacterized in that a fourth conduit is in communication with saidfirst particle inlet and in communication with said lower portion. 8.The apparatus of claim 7 wherein said means for communicating gasdelivers gas to a junction between the bottom of said first conduit andthe top of said second conduit.
 9. The apparatus of claim 8 furthercharacterized in that the length of said first conduit is at least 50%of the combined length of said first conduit, said second conduit, andsaid fourth conduit.
 10. The apparatus of claim 9 wherein the length ofsaid first conduit is at least 80% of the combined length of said firstconduit, said second conduit, and said fourth conduit.
 11. The apparatusof claim 8 wherein the length of said second conduit is less than 3 feet(0.9 meter).